Missile mounted radars used for acquisition and guidance of the missile to a target are frequently of the coherent pulse Doppler radar type. Generally they operate in a search mode to acquire a target and then lock on the target and track it while closing with the target. The searching process is complicated by relatively broad beam-width antennas due to the fact that overall diameters of antennas are limited by the cross-section of the missile, the imposition of clutter signals from the background where the target appears between the missile and the ground or sea, the possibility of an extremely fast closing rate between target and missile when the target and missile are traveling in or nearly in opposite directions, and the high likelihood that target return frequencies will be adjacent to high clutter return signals thereby making acquisition of the real targets even more difficult. Because the vehicle carrying the search radar is moving, ground targets all appear to be moving and have a Doppler return equal to the relative velocity between where the moving radar is looking and the fixed ground. Since the radar antenna does not have zero beamwidth there are quite different angles to different pieces of clutter even within the main beam. As a result the clutter is spread in frequency. This spread is minimized when the radar antenna is pointed along the longitudinal axis of the missile. While the frequency spread may be minimized under these conditions, the frequency shift, or Doppler, will be maximized. Under this condition the spectral width of the return clutter signal is minimum but the Doppler shift is maximum. When the antenna is pointed off to the side, the amount of Doppler shift will be less but the spectral spread will be increased.
Furthermore, signal strength varies according to the orientation of the ground with respect to the radar. If the radar beam looks at the ground and impinges thereon at a 90.degree. angle (perpendicular), the reflected power will generally be maximized. If the impinging radar energy is at a low grazing angle, the return or reflected power will generally be lowered. Another aspect of the flight profile which affects the reflected power is range. It will be readily understood that the reflected power varies inversely with range. There are many combinations of the above conditions which provide a stronger clutter return from the ground than the signal from the target. The probability for this clutter to be within the passband of the radar receiver circuits is high since the radar itself is moving. If the dynamic range of the radar receiver is not able to handle a signal from a very small target simultaneously with that of very large clutter response, the clutter captures the receiver in the radar and drives the target signal down below the threshold device thereby making it unlikely that the radar will respond to the target. Since the radar search filter banks are generally designed to be as wide as possible in order to get as many simultaneous target returns as possible, the probability of having large clutter signals within the passband is enhanced.
Attempts to solve these problems in prior art radar systems have been limited to those which measure clutter over a long period of time in order to average the result. This period of time may be as long as one or two seconds. This averaging technique may not be used in the search portion of a missile guidance system since during that time, because of the extremely high closing rates involved between opposite traveling missile and target, the target might have evaded the missile before lock-on could occur. The long term averaging systems frequently use an AFC loop wherein the loop locks up and moves a filter notch out to the clutter and then dwells there for a very long time, in order to prevent an otherwise very noisy system. In a target acquisition system for a missile, this long period of time is not available. In addition, large frequency spreads are experienced due to the wide antenna beamwidth, and any device which continually blocked out all clutter frequencies would obscure almost all potential target frequencies.
The flight profile wherein the missile radar is looking for a target between the missile and the ground represents a small percentage of the total target volume, but without some system for reducing the clutter effect under these conditions, the acquisition system is not viable.